Semiconductor light emitting device having a silver p-contact

ABSTRACT

A light emitting device is constructed on a substrate. The device includes an n-type semiconductor layer in contact with the substrate, an active layer for generating light, the active layer being in electrical contact with the n-type semiconductor layer. A p-type semiconductor layer is in electrical contact with the active layer, and a p-electrode is in electrical contact with the p-type semiconductor layer. The p-electrode includes a layer of silver in contact with the p-type semiconductor layer. A bonding layer is formed overlying the silver layer to make an electrical connection to the silver layer. The silver layer may be thin and transparent or thicker (greater than 20 nm) and reflective.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.09/212,150, filed Dec. 15, 1998, now U.S. Pat. No. 6,194,793 entitled ANitride Semiconductor Light Emitting Device Having A Silver P-Contact,incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to light emitting electronic devices basedon nitride semiconductors, and more particularly to an optoelectronicdevice that has improved optical and optoelectronic characteristics.

BACKGROUND OF THE INVENTION

The development of short wavelength light emitting devices is of greatinterest in the semiconductor arts. Such short wavelength devices holdthe promise of providing increased storage density for optical disks aswell as full-color displays and white light sources when used inconjunction with devices that emit light at longer wavelengths.

One promising class of short wavelength light emitting devices is basedon group III nitride semiconductors. As used herein, the class of groupIII nitride semiconductors includes GaN, AIN, InN, BN, AlInN, GaInN,AlGaN, BAIN, BInN, BGaN, and BAlGaInN. To simplify the followingdiscussion, “GaN semiconductors” includes GaN, and group III nitridesemiconductors whose primary component is the GaN as in GaInN, AlGaN,BGaN, and BAlGaInN.

Light emitting diodes (LEDs) are fabricated on a GaN semiconductorhaving an active layer that generates light by recombining holes andelectrons. The active layer is sandwiched between p-type and n-typecontacts to form a p-n or n-p diode structure. A p-electrode and ann-electrode are used to connect the p-contact and n-contact,respectively, to the power source used to drive the LED. The overallefficiency of the LED may be defined to be the light emitted to theoutside generated per watt of drive power. To maximize the lightefficiency, both the light generated per watt of drive power in theactive layer and the amount of light exiting from the LED in a usefuldirection must be considered.

A considerable amount of effort has been expended in prior art devicesto maximize the light that is generated from the active layer per wattof drive power. It should be noted that the resistance of the p-typenitride semiconductor layer is much more than the resistance of then-type nitride semiconductor layer. When the p-electrode is formed onthe p-type nitride semiconductor layer, a semiconductor junction orohmic junction is formed. In either case, there is a voltage drop acrossthe junction, and hence, power is wasted at the junction. To reduce thisvoltage drop , the p-electrode is usually much wider than then-electrode to lower the contact voltage.

While increasing the size of the p-electrode increases the amount oflight generated in the active region per watt of input power, it leadsto a decrease in the amount of light that exits the device, since mostof the light exiting the device must now pass through the p-electrode.Accordingly, attempts have been made to maximize the transmittance ofthe p-electrode. A p-electrode having a transmittance of 40 to 50% hasbeen constructed utilizing a multi-layered film of nickel and goldhaving an 8 nm gold film layer on a 1 nm of nickel layer. However, evenwith this relatively high transmittance, there is still considerableroom for improvement.

In addition, this transparent p-electrode is too thin for bonding to theelectrical conductors used to deliver the power to the LED. Hence, athicker p-electrode region is required to form a bonding pad. Amulti-layered film of nickel and gold having a thickness of severalhundreds of nanometers is often used as the bonding pad. The bonding padis typically a rectangle of the order of 100 microns on a side. Hence, asignificant amount of light is lost in the thicker bonding pad regions.

However, even with the best prior art designs, the amount of lightexiting the LED is 50% of that generated in the active region. Ifattempts are made to increase the output by using thinner p-electrodes,the resistance of the electrode increases. As a result higher drivevoltages are required to overcome the increased resistance, andefficiency drops.

Broadly, it is the object of the present invention to provide animproved LED design.

It is a further object of the present invention to provide an LED withincreased light output efficiency.

These and other objects of the present invention will become apparent tothose skilled in the art from the following detailed description of theinvention and the accompanying drawings.

SUMMARY OF THE INVENTION

The present invention is a light emitting device constructed on asubstrate. The device includes an n-type semiconductor layer in contactwith the substrate, an active layer for generating light, the activelayer being in electrical contact with the n-type semiconductor layer. Ap-type semiconductor layer is in electrical contact with the activelayer, and a p-electrode is in electrical contact with the p-typesemiconductor layer. The p-electrode includes a layer of silver incontact with the p-type semiconductor layer. In the preferred embodimentof the present invention, the n-type semiconductor layer and the p-typesemiconductor layer are constructed from group III nitridesemiconducting materials. In one embodiment of the invention, the silverlayer is sufficiently thin to be transparent. In other embodiments, thesilver layer is thick enough to reflect most of the light incidentthereon and light exits via the substrate, which is transparent. Afixation layer is preferably provided over the silver layer. Thefixation layer may be a dielectric or a conductor, the choice dependingon whether or not the silver layer is transparent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an LED according to a firstembodiment of the present invention.

FIG. 2 is a cross-sectional view of an LED according to the presentinvention having a reflective p-electrode consisting of a 3-layeredstructure.

FIGS. 3(A)-(C) are cross-sectional views of an LED and package beforeand after mounting the LED on the package.

FIG. 4 is a cross-sectional view of another embodiment of an LEDaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention achieves its advantages by utilizing asilver-based p-electrode. An LED according to the present invention canbe constructed with either a reflective p-electrode and a transparentsubstrate or a transparent p-electrode and a reflective substrate.Embodiments of the present invention utilizing a reflective p-electrodewill be described first.

The light generated in the active region of the LED exits the LEDthrough the substrate when a reflective p-electrode is utilized. Lightexiting the active region in the direction of the p-electrode isreflected back toward the substrate by the reflective p-electrode. Thep-electrode utilizes silver as the first metal layer in at least a partof the p-electrode. In an optoelectronic device according to the presentinvention, a silver layer is vapor-deposited on the p-type nitridesemiconductor layer and functions as the p-electrode and as a mirror forreflecting light back toward the transparent substrate.

Refer now to FIG. 1, which is a cross-sectional view of an LED 10according to a first embodiment of the present invention. LED 10 isconstructed on a sapphire substrate 2 by depositing an, n-layer 3, anactive layer 4 that is usually a multi-quantum well layer of a nitridesemiconductor, and a p-layer 5. This series of layers is essentiallythat deposited in constructing a conventional LED up to the point ofproviding the p-electrode. LED 10 utilizes a silver (Ag) layer 21 as thefirst metal layer of the p-electrode. A second metal layer for bondingthe p-lead wire 6 is constructed from nickel and gold and shown at 21A.In some embodiments, the bonding layer 6 covers less than half of silverlayer 21. An n-electrode 7 is also provided in the conventional mannertogether with a bonding pad for connecting the n-lead wire 7A.

The method by which LED 10 is fabricated will now be explained in moredetail. First, conventional processes such as CVD are used tosuccessively form n-layer 3, active layer 4, and p-layer 5 on a sapphiresubstrate 2. Next, the LED is patterned photolithography using nickel asthe mask and is etched back into n-layer 3 to form the pad forn-electrode 7 by reactive ion etching. The nickel mask is then removedby applying aqua regia at room temperature.

The removal of the mask via aqua regia also cleans the surface ofp-layer 5, and hence, aqua regia is preferred to other etchants forremoving the nickel mask. The part is left in the aqua regia for 30minutes to one hour. If the etching time is less than 30 minutes, thecleaning of the p-layer surface is insufficient, even though the nickelmask has been removed. Insufficient cleaning leads to a loss in thestability of the silver that is vapor-deposited on p-surface in thesubsequent deposition steps. Hence, reducing the immersion timesignificantly below 30 minutes must be avoided.

Next, the LED part is activated for five minutes with sapphire substrate2 at 900° C. in a nitrogen atmosphere. After the activation, the LEDpart is cleaned in hydrofluoric acid for 10 minutes at room temperature.A 100 nm layer of Ag is then vapor-deposited on p-layer 5 to form thefirst layer 21 of the p-electrode. It should be noted that thereflectance of the Ag layer does not improve substantially if thethickness is increased above 100 nm.

Next, about 300 nm of nickel and 50 nm of gold are successivelyvapor-deposited and patterned to form electrode metal layer 21A forbonding to the p electrode and a first annealing is performed (annealing1). Other metals that can be used to form metal layer 21A are aluminumand indium.

Next, 10 nm of Ti and 200 nm of Al are successively vapor-deposited andpatterned on the n-type GaN part to form n-electrode 7. A secondannealing operation is then performed. The LED may then be separatedfrom the other devices, such as other LEDs, formed on the same wafer.The LED is then mounted in a suitable package (not shown), the p-leadwire 6 is connected between the electrode metal layer 21A and a firstbond pad (not shown) that forms part of the package, and the n-lead wire7A is connected between the n-electrode 7 and a second bond pad (notshown) that forms part of the package. The LED is oriented in thepackage in a direction that allows light transmitted through thesubstrate 2 to be radiated from the package.

It should be noted that annealing 1 can be omitted. Annealing 1 isperformed at or below 200° C., and annealing 2 is performed above 200°C., preferably above 400° C. The annealing operations are foundexperimentally to reduce the resistance of the p-contact.

The characteristics of an LED according to the present invention dependon the speed with which the silver is deposited and on the temperatureof the sapphire substrate during the vapor deposition. It has been foundexperimentally that the preferred deposition conditions are a vapordeposition speed of approximately 0.05 nm/second or less and atemperature of the sapphire substrate 2 of 200° C. or less. Attemperatures of 400° C., the silver layer becomes non-uniform, and theresistance of the silver layer increases. As noted above, the resistanceof the p-electrode is a significant factor in the overall efficiency ofthe LED, and hence, such increases in resistance are to be avoided.

The silver-based p-electrode of the present invention is particularlywell suited for reflective electrodes in the blue to green region of thespectrum. While palladium, platinum, nickel, gold, aluminum, chromium,and titanium layers could be utilized to create a reflective electrode,silver has a substantially higher reflectance than the other candidates.In addition, silver, unlike gold, aluminum, chrome, and titanium, formsan ohmic junction at the p-type GaN.

The portions of the silver layer that are not covered by the mountingpad 21A are preferably covered by a fixation layer that preventsreductions in the reflectance of the Ag layer over time. The fixationlayer can be a metal or a dielectric. Examples of suitable metalsinclude nickel, palladium, and platinum.

In one embodiment, the fixation layer is a metal layer that covers theentire silver layer and acts as a passivation layer that prevents thediffusion of the metal from the bonding layer into the silver layer.Refer now to FIG. 2 which is a cross-sectional view of an LED 15according to the present invention having a reflective p-electrodeconsisting of a 3-layered structure that includes a diffusion barrierlayer 102. To simplify the discussion, those elements that serve thesame functions in LED 15 as elements in LED 10 shown in FIG. 1 have beengiven the same reference numbers. Silver layer 101 is covered with adiffusion barrier layer 102. Diffusion barrier layer 102 is covered bythe bonding layer 103 to which wire or other bonding connections aremade.

Diffusion barrier layer 102 prevents the constituents of bonding layer103 from diffusing into silver layer 101. The diffusion barrier layer ispreferably constructed from nickel and is vapor-deposited to a thicknessup to 300 nm. In the preferred embodiment of the invention, diffusionbarrier layer 102 also covers the side surfaces of silver layer 101 andseals the p-layer 5 and the silver layer 101. However, this sealingfunction may not always be required. Next, the metal for the bondinglayer 103 is vapor-deposited. Gold with a thickness of 50 nm ispreferred.

In the absence of a diffusion barrier layer, gold from the bonding layerdiffuses into the silver layer and reduces the reflectivity of thesilver layer. If the bonding layer covers only a small fraction of thep-electrode, the reduction in the overall reflectivity of thep-electrode is relatively small. However, when the bonding layeroccupies a significant fraction of the p-electrode, the reduction inreflectivity is significant, and hence, the diffusion barrier layerprovides a significant improvement.

The diffusion barrier layer 102 also improves the stability of theunderlying silver layer 101. The diffusion barrier layer also functionsas a metal fixation layer that improves the mechanical and electricalcharacteristics of the underlying silver layer. As a result of theseimprovements, the substrate temperature during the vapor deposition stepin which the silver layer is formed can be lowered and the vapordeposition speed increased.

The above-described embodiments of the present invention utilized singlelayers for the diffusion barrier layer 102 and the bonding layer 103.However, it should be noted that the diffusion barrier layer 102 and/orthe bonding layer 103 can be multi-layered structures.

The reflective p-electrode discussed above is well adapted for LEDs thatare to be “flip-chip” mounted. Refer now to FIGS. 3(A)-(C), which arecross-sectional views of an LED 130 and package 140 before and aftermounting LED 130 on package 140. LED 130 is constructed on a transparentsubstrate 132. The p-electrode of LED 130 is shown at 113 and includes abonding layer or bonding pad as described above. The n-electrode isshown at 117. In the packaging operation, p-electrode 113 is to beconnected to a conductor 118 on the package, and n-electrode 117 is tobe connected to conductor 119.

The bonding layer 103 shown in FIG. 2 is chosen to provide a compatiblesurface for making electrical connections between the electrodes and theconductors in the package. The connections are provided by depositing“bumps” 120 of a low melting-point metal such as indium on the packageconductors. Similarly, a coating of a metal that will wet the lowmelting-point metal is deposited on the surface of the n-electrode andp-electrode as shown at 116. LED 130 is then inverted and placed incontact with package 140. The parts are then heated sufficiently to meltthe low melting point metal thereby making the required connections. Thefinal packaged part is shown in FIG. 3(C).

The above-described embodiments of the present invention have utilized areflective silver-based p-electrode with the light exiting the LEDthrough a transparent substrate. However, embodiments of the presentinvention having a transparent p-electrode can also be constructedutilizing silver. Refer now to FIG. 4, which is a cross-sectional viewof another embodiment of an LED according to the present invention. Tosimplify the following discussion, the same reference numbers areutilized for parts that serve the same function as parts shown in FIG.1. LED 50 differs from LED 10 shown in FIG. 1 in that silver layer 51 isthinner than silver layer 21 shown in FIG. 1, and a TiO₂ layer 52 isdeposited on the p-electrode. Layer 52 helps to protect and stabilizethe silver layer. In addition, TiO₂ layer 52 reduces the reflectance ofthe p-electrode as described below. The thinner silver layer improvesthe transmittance of the p-electrode.

Electrode metal layer 51A provides a bonding pad for connecting thep-electrode wire 6. This pad is similar to the electrode metal layer 21Ashown in FIG. 1.

In this embodiment of the present invention, silver layer 51 has athickness of 3 to 20 nm, preferably 10 nm. A silver layer becomestransparent when its thickness is less than 20 nm. It should be notedthat at wavelengths below 500 nm, the absorption of silver is less thanthat of gold. Hence, this embodiment of the present invention is usefulin constructing short wavelength LEDs with transparent p-electrodes.

It should be noted that a combined TiO₂(25 nm)/Ag(10 nm) film has ahigher transmittance at wavelengths above about 360 nm than a singlesilver film. Hence, the TiO₂ film also improves the transmission of thep-electrode. The optimum thickness for the TiO₂ film depends on thewavelength of the light generated by the LED. The TiO₂ film provides anoptical matching layer that reduces the reflections from the silverlayer. The optimum thickness of the TiO₂ layer is independent of thethickness of the silver layer and is approximated by 25λ/450, where λ isthe wavelength (in nm) of the generated light.

TiO₂ layer 52 is preferably deposited by vapor-deposition. When TiO₂layer 52 is used, the conditions under which silver layer 51 isdeposited are less critical than described above with respect to LED 10shown in FIG. 1. In particular, the vapor deposition speed of silver canbe increased.

It should be noted that other dielectric films may be used in place ofTiO₂. For example, layer 52 may be constructed from SiO₂ or Al₂O₃.

In the preferred embodiment of the present invention, either theboundary of substrate package 8 and substrate 2 or the boundary ofsubstrate 2 and n-layer 3 is reflective for light of the wavelengthgenerated in active layer 4. Such a reflective layer assures that lightleaving active layer 4 toward substrate 2 is reflected back toward thetransparent p-electrode.

The above-described embodiments of the present invention have utilized asapphire substrate. However, it will be obvious to those skilled in theart from the preceding discussion that other substrates may be utilized.In addition, the substrate may include one or more buffer layers, then-type semiconductor layer being deposited on the last of these bufferlayers. Accordingly, it is to be understood that the term “substrate”includes such buffer layers.

Similarly, the above-described embodiments of the present invention havebeen described in terms of a p-type semiconductor layer and an n-typesemiconductor layer that sandwich an active layer that generates lightwhen a potential is applied across the semiconducting layers. However,it will be obvious to those skilled in the art from the precedingdiscussion that each of these layers may include a number of sub-layers.Accordingly, it is to be understood that the term “layer” as used hereinincludes multi-layered structures.

Various modifications to the present invention will become apparent tothose skilled in the art from the foregoing description and accompanyingdrawings. Accordingly, the present invention is to be limited solely bythe scope of the following claims.

1. A light emitting device comprising: a substrate; an n-typesemiconductor layer; an active layer for generating light, said activelayer being in electrical contact with said n-type semiconducting layer;a p-type semiconductor layer in electrical contact with said activelayer; and a p-electrode in electrical contact with said p-typesemiconductor layer, said p-electrode comprising: at least a layer ofsilver having a thickness sufficient to reflect greater than 50% oflight incident thereon, wherein a portion of said generated light exitssaid device through said substrate after being reflected from saidp-electrode; a bonding layer in electrical contact with said layer ofsilver for making electrical connections to said layer of silver; and afixation layer overlying at least a portion of said layer of silver,wherein the fixation layer is conductive.
 2. The light emitting deviceof claim 1 wherein said n-type semiconductor layer and said p-typesemiconductor layer comprise group III nitride semiconducting materials.3. The light emitting device of claim 1 wherein said silver layer isgreater than or equal to 20 nm in thickness.
 4. The light emittingdevice of claim 1 wherein said fixation layer comprises a metal.
 5. Thelight emitting device of claim 4 wherein said fixation layer comprises ametal chosen from the group consisting of nickel, palladium, andplatinum.
 6. The light emitting device of claim 1 wherein said bondinglayer comprises a metal chosen from the group consisting of gold,nickel, aluminum, and indium.
 7. The light emitting device of claim 1wherein said bonding layer covers less than half of said layer ofsilver.
 8. The light emitting device of claim 1 wherein said bondinglayer is a multi-layered structure.
 9. The light emitting device ofclaim 1 wherein said fixation layer is disposed between said bondinglayer and said layer of silver, said fixation layer providing anelectrical path between said bonding layer and said layer of silver,said fixation layer serving as a diffusion barrier layer for preventingconstituents from said bonding layer from interdiffusing with said layerof silver.
 10. The light emitting device of claim 9 wherein saidfixation layer comprises a metal.
 11. The light emitting device of claim10 wherein said fixation layer comprises nickel.
 12. The light emittingdevice of claim 9 wherein said fixation layer encapsulates said layer ofsilver.
 13. The light emitting device of claim 1 further comprising: ann-electrode comprising a layer of electrically conducting material inelectrical contact with said n-type semiconductor layer; and a packagehaving first and second conductors thereon electrically connected tosaid p-electrode and said n-electrode, respectively.
 14. A lightemitting device comprising: a substrate; an n-type semiconductor layer;an active layer for generating light, said active layer being inelectrical contact with said n-type semiconducting layer; a p-typesemiconductor layer in electrical contact with said active layer; and ap-electrode in electrical contact with said p-type semiconductor layer,said p-electrode comprising: at least a substantially transparent layerof silver; a bonding layer in electrical contact with said layer ofsilver for making electrical connections to said layer of silver; and afixation layer overlying said layer of silver, wherein the fixationlayer is conductive.